<!DOCTYPE html> 
<html lang="en"> 
<head> 
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width,initial-scale=1.0" charset="UTF-8" />
    <title>惰性函数与性能</title> 
</head> 
<body>
    <button id="btn">
        点击
    </button>
<script type="text/javascript">
    var oBtn = document.getElementById('btn')

    function foo(){
        console.log(this)
    }
    // function addEvent(obj,type,fn){
    //     if(obj.addEventListener){
    //         obj.addEventListener(type,fn,false)
    //     }else if(obj.attachEvent) {
    //         obj.attachEvent('on' + type,fn)
    //     } else {
    //         obj['on' + type] = fn
    //     }
    // }

    function addEvent(obj,type,fn){
        if(obj.addEventListener){
            addEvent = obj.addEventListener(type,fn,false)
        }else if(obj.attachEvent) {
            addEvent = obj.attachEvent('on' + type,fn)
        } else {
            addEvent = obj['on' + type] = fn
        }
        return addEvent
    }

    addEvent(oBtn,'click',foo)
</script> 
</body> 
</html>